Lead is a toxic metal which can be harmful to human health even at low exposure levels. Lead is sometimes referred to as a cumulative toxin because the lead concentrates in the body. Young children, infants, and fetuses are particularly vulnerable to lead because the physical and behavioral effects of lead occur at lower exposure levels in children than in adults. Overexposure to lead can permanently impair a child's mental and physical development.
Comparatively low levels of exposure have been linked to damage to the central and peripheral nervous system, learning disabilities, shorter stature, impaired hearing, and impaired formation and function of blood cells. At its worst, lead poisoning can result in stupor, coma, kidney damage, or severe brain damage.
Lead in drinking water (i.e. potable water) can be a significant contributor to overall exposure to lead, particularly for infants whose diet consists of liquids made with water, such as baby food formula. Consequently, there is a great need to test potable water to determine whether such water contains less than the 5 parts per billion (ppb) maximum limit proposed by the EPA for public drinking water and to check for less than 10 parts per billion on firstdraw samples at the point of use. One ppb is equal to one microgram per liter.
Although it is possible to analyze water samples using atomic absorption (i.e. spectroscopic analysis) techniques, such techniques are cumbersome and subject to error due to interferences. For example, the sample must first be prepared in order to convert organic forms of lead to inorganic forms, to minimize organic interferences, and to convert the sample to a suitable solution for analysis. Then the prepared sample is placed into a graphite tube furnace where the sample is slowly evaporated to dryness, charred (ashed) and then atomized. The absorption of hollow cathode radiation during atomization is proportional to the lead concentration.
The atomic absorption method is subject to various disadvantages. For example, it requires the use of an atomic absorption spectrophotometer and a graphite furnace. It also requires a trained operator, a lengthy set-up time, and the equipment required is extremely expensive. Thus, this technique is not suitable for use in the field. Rather it must be used in the laboratory.
Also, the atomic absorption method is subject to various types of interference. The long residence time and high concentrations of the atomized sample in the optical path of the graphite furnace can result in severe physical and chemical interferences. Furnace parameters must be optimized to minimize such effects. Lead analysis can also suffer from severe nonspecific absorption and light scattering caused by matrix components during atomization. Simultaneous background correction must be employed to avoid erroneously high results. Also, if the analyte is not completely volatilized and removed from the furnace during atomization, memory effects will occur, thereby requiring cleaning of the tube by operating the furnace at higher atomization temperatures. Further, the presence of sulfate can suppress lead absorbance, thereby requiring the use of a lanthanum releasing agent.
It is also possible to analyze potable water for lead using ammoniacal citrate-cyanide reducing solution, followed by extraction with dithizone in chloroform. The final solution can then be analyzed photometrically. This method is not reliable for detecting low levels of lead in water. Also, the use of solvents and cyanide presents a disposal problem. Thus, this method is not readily useful except in a laboratory environment.
The use of special ion exchange resins, such as the phosphoryl-modified cellulose based "Cellex P", has been described, but such use requires very carefully controlled flow rates for passing fluid solutions through the material in order to retain the metals on the resins. Such materials are of small particle size (measured in microns). Oftentimes the flow rates are extremely slow (e.g., 1-5 mL per minute) which makes the analysis very time consuming. Also, the required elution step involves the use of strong acids or specific flow rates to separate the metals. As a result, the use of ion exchange resins has attendant drawbacks.
Solvent extractions have also been used in separation of lead ions from solution. Such extractions involve the use of amine complexing agents and toxic or flammable reagents. Also, further concentration, drying or ashing of the extract is often necessary before analysis can be completed.
There has not heretofore been described a simple, effective, rapid, and safe technique for extracting lead ions from potable water and then analyzing to determine the amount of lead extracted.